Crystallographic detection of low-frequency motions in proteins: the breathing of the β-sheet core in RNase A

X-ray crystallography is the most accurate physico-chemical method for determining the 3D structure of proteins. This method can be used also to provide important information about the dynamic aspects of a molecular structure, such as thermal displacement parameters or dynamic disordering effects. In addition, in this paper we show that low frequency motion can also be analyzed by this technique, provided that multiple structural models of the same protein, crystallized in different crystal environments, are available. For bovine pancreatic ribonuclease (RNase A), a large number of crystallographic studies have been carried out on crystals obtained in a variety of experimental conditions. These structural data were used to map the low-frequency β-sheet closure motion of this protein, which is considered to be important in determining its enzymatic activity. The force constant and the frequency derived from the X-ray crystallographic data are in good agreement with the values obtained from other experimental techniques.